1. Field of the Invention
This invention relates to an improvement in and relating to an extruder with a roller die.
2. Description of Prior Art
In a conventional extruder with a roller die, material which is discharged from a batch type mixer is fed into a chute of a hopper and forced into a cylinder by a pusher. The material is further fed forward by a screw which is rotated in the cylinder, toward a bank portion of a pair of horizontally disposed roller dies which are located at the front end of the extruder, and rolled into a sheet. The rear or base end of the screw is keyed or splined to and rotatingly driven by an output shaft of a reducer while the fore end portion of the screw is contacted and supported by the inner periphery of the cylinder. There arises no problem as long as the screw is maintained exactly in the center position within the cylinder and the trough of the screw is uniformly loaded with the feed material. However, in the actual operations, such an ideal condition is not expected and, in spite of antifrictional treatment, the fore end portion of the screw is subjected to considerable abrasive wear due to contact with the inner periphery of the cylinder during rotation. Therefore, early repair and replacement of parts are required in the maintenance and service of the machine.
In the conventional extruder of this class, the screw usually has a feed portion, a compression portion and a metering portion, such metering portion serving to generate a high pressure in the fore end portion of the extruder and extrude the material at a constant rate against the generated pressure and without pulsations. Therefore, the metering portion has been considered to be essential to extruders. However, in an extruder with a roller die, the fore end of the extruder is opened into the bank portion of the roller die, so that there is no necessity for correctly metering the feed material and it suffices to maintain a feed pressure and an accuracy which ensure the feed material to be rolled into a continuous sheet by the roller die. That is to say, the metering function is not required as strictly as in the extruders of other types (e.g., extruders with a pelletizer or a sheeting die). Therefore, an extruder with a roller die can dispense with the metering portion if the rolling and extruding speeds are controlled in such a manner as to ensure continuous sheet formation on the basis of the amount or pressure of the bank of the roller die.
The extruding material within the extruder has a pressure distribution as shown in the graph of FIG. 2. It is known therefrom that the discharge capacity of the screw which is fully loaded with the material is not constant and gradually reduces forwardly in the compression portion. On the other hand, the flow rate of the loaded material tends to be reduced and increased on the sides of the larger and smaller discharge capacities, respectively, under the influence of the pressure difference thereby maintaining the continuity of the flow rate. Normally, the pressure P.sub.h at the fore end of the cylinder is smaller than the maximum pressure P.sub.max which is generated by the difference in discharge capacity, the pressure decreasing linearly therebetween showing a pressure distribution curve as in FIG. 2. As for an operating range, such is conditional to maintain the pressure at the fore end of the cylinder at a level between a minimum allowable value for the sheet formation and a maximum allowable value in terms of the mechanical strength of the extruding material, and to maintain the maximum pressure P.sub.max at a level between a minimum allowable value which keeps the sheet free of pores and bubbles and a maximum allowable value in terms of the mechanical strength. These pressures are important factors which influence the quality of the sheet to be produced. The pressures P.sub.h and P.sub.max are varied depending upon the viscosity of the extruding material, normally being in proportion thereto. Nevertheless, no attention has been paid to the control of these pressures in the conventional extruders of the class mentioned above.